This invention relates generally to digital voice communication and more particularly to a multirate digital communication processor which provides an improved quality of speech at a data rate of 9.6 kilobits per second (kbps), and interoperability among systems that operate at 2.4 kbps and 9.6 kbps.
Voice communications are being increasingly encoded by digital, rather than analog, techniques because digital encryption for security purposes is both easier and less vulnerable to unauthorized decryption.
The choice of a data rate for processed speech is dependent upon such factors as speech quality requirements, availability of transmission channel bandwidth, availability of equipment, and leased-line costs. Although wideband systems which operate at 32 kbps or greater have virtually no degradation in speech quality, availability of these wideband trunks and access lines is limited. Therefore, some users must rely on narrowband channels. A solution is to use two different voice processors, one for wideband at 16 kbps/32 kbps and the other for narrowband at 2.4 kbps. However, there must be a tandeming process to allow the two incompatible voice processors to communicate with each other. Existing tandeming processes require digital-to-analog and analog-to-digital conversions. These conversions degrade speech intelligibility and quality. Degradation is even more severe if speech is contaminated by background noise. More importantly, regarding secure voice communications, the presence of an analog interface prevents end-to-end security from being achieved since speech data must be decrypted at the tandem junction. The present invention generates a data stream of 2.4 kbps which is embedded within a data stream of 9.6 kbps. This embedded data structure eliminates the need for analog tandeming between high- and low-data-rate users because the rate conversion is effected directly on the bit stream.
A multirate processor for generating an embedded data stream of 2.4 kbps within 9.6 kbps cannot be formed by combining a 2.4 kbps voice processor and a 7.2 kbps voice processor, because existing 7.2 kbps processors are not capable of producing an acceptable quality of speech.
In addition, existing 9.6 kbps processors are not adaptable for changing the data rate without extensive and complex modifications.